Clutch-clutch-brake steering mechanism for tractors

ABSTRACT

Clutch-clutch-brake steering unit having hydraulically actuated cylinders for controlling same to control rotation of a track-drive sprocket in a crawler tractor. Specifically, one of the clutch cylinders and a brake cylinder provided in the unit have an inlet-outlet port in common, and are so arranged on a spring-applied-brake, hydraulically-applied-clutch basis that a single pressure signal in the inlet-outlet port alternately operates the cylinders to apply the brake and release the one clutch, to apply the one clutch and release the brake or, by proper modulation, to partially engage either, i.e., slip the clutch or drag the brake to intermediate degrees as desired. And, to nearly the same extent, the other clutch cylinder and the brake cylinder provided in the unit have inlet-outlet ports in common and are similarly arranged that the absence or presence of a single pressure control signal in their inlet-outlet ports alternately operates the cylinders to apply the brake and release the other clutch, or to apply the other clutch and release the brake.

This is a division of application Ser. No. 176,815, filed Aug. 11, 1980,now U.S. Pat. No. 4,372,408, which is a continuation of Ser. No. 951,690filed Oct. 16, 1978, and now abandoned.

Also, it is a companion case to Chatterjea et al. U.S. Pat. No.4,015,619 and to Chatterjea et al. U.S. Pat. No. 4,164,276, assigned tothe same assignee.

The present application relates to clutch-clutch-brake mechanism for usein vehicles which are steered-by-driving. More particularly, theapplication relates to the left and right clutch-clutch-brake steeringunits for a crawler tractor. Left and right clutch-brake steering unitsare disclosed in the companion crawler tractor patents noted, U.S. Pat.Nos. 4,015,619 and 4,164,276, which have only steering brake turns withno geared steering; whereas the second clutch of the clutch-clutch-brakemechanism hereof superimposes the geared-steer option affording, alongwith other versatility of significance hereinafter emphasized, therub-free type of full power turns, left and right with no rubbing andfriction losses in the friction clutches which are not allowed to slipand no rubbing and friction losses in the steering brakes which are notallowed to drag while steering with gearing.

Background patents further include but are not limited to U.S. Pat. Nos.1,835,790, 2,323,115, 2,447,920, 2,974,639, 2,984,213, 3,215,044,3,220,318, 3,221,770, 3,460,577, 3,540,559, 3,677,362, 3,693,503,3,706,322, 3,757,915, 3,843,205 and particularly Bidwell 3,018,041.

It is an object in connection with steering mechanism according to myinvention, to provide an alternately acting hydraulically actuatedclutch-clutch-brake unit so arranged that braking is automatic (springapplied) upon failure of hydraulic pressure, thus affording fail-safebraking.

An object in line with the preceding objective is to provide aclutch-clutch-brake unit in which one clutch and brake aresiamese-connected for pressure actuation of same, and in which a singlesignal is therefore usable for control pressure both for the one clutchand for the brake in the unit; provision is made in the same way forcontrol pressure that is either equal or unequal to the single signalabove both for the other clutch and for the brake in the unit. One suchclutch constitutes the hi speed clutch providing for a high gear drive,and the other such clutch constitutes a lo speed clutch providing for alow ratio gear drive through the steering unit.

A further object is to provide a metering valve for controlling thesignal pressure, also provide an inlet-outlet port to which the loclutch and brake cylinders are connected in common and controlled by themetering valve as the signal pressure is applied or released, andadditionally provide correlated sets of springs on the lo clutch andbrake causing engagement of each cylinder to be cushioned by delayedpressure-change owing to the fact that the volume of the cylinder of theother is undergoing a metered change because of the metering valve'srestrictive control over the common inlet outlet port.

Also, even though the gear ratio is fixed in high gear, and fixed in lowgear, and the high gear clutch is unmodulated, there is an additionalobject in connection with the lo clutch and brake cylinders. Pressure totheir aforesaid inlet-outlet port is modulated with the object to slipthe lo speed clutch for controlled speeds in the upper half of the lowspeed range and to drag the brake for controlled speeds in the lowerhalf of the low speed range.

An additional object is to provide consolidated valving in a housing sosimplified that a minimum number of spool bores are provided, and incompact manner and with internal interconnections of minimum length.

According to practice in the past in connection with someclutch-clutch-brake steered crawler tractors, the steering clutch hasbeen operated through a controlled rate-of-rise valve to cushion clutchengagement. Similarly the companion brake has been operated through asecond valve and, if the manner desired has been to cushion the brakeengagement, the second valve has likewise afforded controlled rate ofpressure rise in the brake cylinder. It has therefore been the practiceto have some accurate coordinating means providing for precisecoordination of operation of the two clutch and single brake valves,which only slightly overlap in operation so as to ensure appreciablereleasing of the engaged clutch prior to starting to engage the brake,and vice versa. Disadvantages and inherent complications have arisenbecause of the need for special rate-of-rise valve control andcoordination of the valve operations in controlling or slowing rate ofpressure rise in the cylinders.

My invention materially reduces, if not substantially eliminating, theforegoing disadvantages and complications, as will now be explained indetail. Various features, objects, and advantages will either bespecifically pointed out or become apparent when, for a betterunderstanding of my invention, reference is made to the followingdescription taken in conjunction with the accompanying drawings whichshow a preferred embodiment thereof and in which:

FIG. 1 is a schematic view, in top plan, of a crawler tractor embodyingour hydraulically actuated, clutch-clutch-brake invention;

FIG. 2 is a partially schematic, cross sectional view of aclutch-clutch-brake mechanism and hydraulic control circuit therefor;

FIGS. 3 and 4 are graphs showing the desired pressure time tracesinvolved in achieving respective cushioned clutch application andcushioned brake application; and

FIGS. 5, 6, 7, and 8 are partly schematic, cross sectional views of asteering valve, showing it in various positions in its operation in thehydraulic control circuit of clutch-clutch-brake mechanism.

More particularly, in FIG. 1 of the drawings, a crawler tractor 10 isshown having undercarriage structure including right and left endlesstrack assemblies 12 and 14, having a front mounted engine 16, and havinga chassis, not shown, supported on the undercarriage structure andsupporting the engine 16. The undercarriage structure further includesat its respective sides right and left front idler wheels 18 and 20 andright and left track drive sprockets 22 and 24.

The power train from the engine 16 of the tractor includes a threespeed, reversible power shift transmission 26, a rear main frame 28holding the steering mechanism, and suitable interconnections in thepower train whereby the sprockets 22 and 24 receive their drivingtorque, as from a right final drive 30. More specifically, a torqueconverter 32 interconnects the engine 16 and the power shifttransmission 26. Meshing bevel and crown gearing 34 interconnects thetransmission 26 and the rear axle 36 of the tractor, and a rightsteering clutch-clutch-brake unit 38 controls power rotation for brakingof the right final drive 30 which is supplied with torque by the rearaxle 36.

For purpose of hydraulic control over the steering mechanism in the rearmain frame 28, the tractor 10 has a steering valve assembly 40 and, forthat purpose, the steering valve assembly has operator-operated handlevers including a right steering lever 42 connected to a right steerspool valve 44 included int he valve assembly 40. A brake pedal 46 isconnected to a brake spool valve 48 invluded in the valve assembly 40.

The steering valve connections are omitted from FIG. 1 forsimplification, but receive the usual hydraulic fluid for theiroperation from a rotary pump 50 driven by a meshing pinion and gear 52which are connected to the output side of the torque converter 32. Aregulator valve 54 which is teed to the output side of the pump 50 has aconstant setting so that pressure in the output line 56 will beessentially constant, e.g., 290 psi (20 atmospheres) under varying pumpspeeds.

The steering controls and drives in the tractor 10 are essentiallysymmetrical, thus further including, for the left side, a secondsteering lever 42-2, a second steering valve 44-2, a secondclutch-clutch-brake unit 38-2, and a second final drive 30-2. The valves44 and 44-2 are modulator valves essential to the modulationaccomplished in the steering valve assembly 40 referred to in theparagraph following; also, the steering units 38 and 38-2 constitute amulti-ratio drive and braking train essential to my purposes inproducing the overall operation now to be described.

In operation of the tractor 10, torque in the power train from theengine 16 is applied through the torque converter 32 to the power shifttransmission 26 and gearing 34, which drive the rear axle 36 at a speedand in the direction selected. Line pressure from the line 56 is appliedwith appropriate modulation by the steering valve assembly 40 to theclutch clutchbrake units 38 and 38-2 so that the operator can cause thesprockets 22 and 24 to rotate the endless track assemblies 12 and 14 atthe same speed for straight line drive in either direction, or atdifferent speeds for power turns or braked turns, either left or rightforwardly, or left or right rearwardly.

STEERING MECHANISM-- FIG. 2

In accordance with the arrowed path appearing in this figure, power flowfrom the transmission, not shown, follows along a propeller shaft in thepath of an arrow 58 into the bevel and crown gearing 34 so as to beconducted laterally by the rear axle 36 in the opposite directionsindicated by a double headed arrow 60. In the right direction, forexample, lateral power flow from the axle and right steeringclutch-clutch-brake unit 38 is in the direction of an arrow 62, throughbull gearing 64 and the right final drive 30, not shown, thence into theright track drive sprocket 22. More particularly, except for a radialflange 66 integrally carried by the axle adjacent its right end, theaxle 36 which is hollow is generally symmetrical when one end iscompared to the other. The axle 36 is journalled for rotation in aclutch-clutch-brake housing 68 by means of a span of bearings includinga tapered roller bearing 70 illustrated. Longitudinal passages in theaxle 36 include separate, oppositely extending hydraulic passages 72 and74 which are parallel to, and hydraulically separated by pressed-intubes from, the hollow interior 76 thereof. The referred to radialflange 66 at its periphery carries a crown gear 78 meshing with thebevel pinion of the gearing 34 and driving the axle 36 through a desiredreduction gear ratio.

A bore 80 in the end of the axle 36 receives a pressed-in sleeve 82, andthe sleeve 82 transmits to lube passages 84 and 85 bearing lubricantsupplied to the hollow interior 76.

A right-steer brake includes an hydraulic brake piston 86B sealedwithin, and adjacent, a steering drive cover 88, a back plate 89, abrake cylinder 90B, a splined member 91 grounded by the cover to thesteering gear casing, and brake springs 92 for the spring-applied,pressure-released brake.

Selective gear ratio clutch actuations are afforded by a lo clutchpiston 94C and cylinder 96C, and by a hi clutch piston 95H and cylinder97H.

The counterpart left-steer cylinders for braking, lo clutching, and hiclutching appear at 90B-2, 96C-2, and 97H-2.

The lo clutch includes clutch release springs 98 and a backing plate102. A planetary gearset to provide the lo clutch ratio drive (e.g., a1.3:1 gear reduction) within the clutch housing 100 includes an inputring gear 104 carried by the housing 100 and a sun gear reaction member106 therewithin. An output shaft 112 coaxial with axle 36 issleeve-splined to an output carrier 114 in the planetary carrying a setof dual planets 116.

The smaller set of pinion teeth on each dual planet 116 continuallymeshes with the input ring gear 104, and the larger set of pinion teethcontinually meshes with the reaction sun 106. The sun 106 is grounded tothe case by a disk stack 110C in the modulated lo clutch when the latteris actuated, whereupon the ring to planets to carrier drive causes thelo ratio speed output desired from the output shaft 112.

The sun reaction member 106 will be free of restraint, whereas theoutput carrier 114 is grounded to the case by a disk stack 108B in themodulated brake when the latter is actuated, whereupon motion of theoutput shaft is either arrested or braked to a bolt and, in the idle setof planets 116, each one is freely rotated, unloaded, on its orbiting orbraked-stationary axis.

Finally, the sun reaction member 106 and output carrier 114 will haveconjoint rotation and the planetary gearset will be locked up, whereasthe input ring member 104 and the output carrier 114 are directlyclutched together by a disk stack 111H in the unmodulated hi clutch whenthe latter is actuated, whereupon the locked planetary provides a 1:1direct drive between the input ring 104 and output shaft 112.

The carrier 114 and output shaft 112 share the common output axis 118defined by their bearings 120, 122, and 124 in the housing 68.

A stationary, case mounted lube oil header 126 supplies lubricating oilin customary way to the rotating axle 36 through a hole 128 in thelatter leading into its hollow interior 76. In similar manner, hi clutchheaders 130 and 131 supply control oil to their clutches respectively ina path including passage 72, thence a passage 132, and into a right hiclutch cylinder 97H, and in a path including passage 74, thence apassage 133, and into a left hi clutch cylinder 97H-2.

The left clutch cylinder 97H-2 is under hydraulic control, through theaforesaid path, from valving hereinafter described and operating in asecond bore 190-2 of the valve assembly 40 which can be viewed overall,with respect to its totality of bores and purposes, as a consolidatedvalving assembly means.

STEERING OPERATORS-- FIG. 2

Hand and foot operation is utilized according to the illustrated examplein FIG. 2, and the operators comprise the brake pedal 46, and the leftand right steering levers 42-2 and 42. The brake pedal 46 has a bottompivot 134 and, through a clevis connection to a mid-pin 136 on the pedalarm, the pedal 46 operates a pull connection 138 to pull the foot brakespool valve 48 to a braking position as opposed by spool valve returnspring means 140. The brake-applied position of the pedal is shown inbroken lines in FIG. 2.

The steering levers have a mid-pivot such as illustrated at 142 for theright steering lever 42 and, as shown in FIG. 2, the connection from thesteering lever 42 is schematically illustrated at 144 whichinterconnects with the spool valve 44 to pull it into the brakedposition as opposed by spool valve return spring means 146. Orespecially as can be seen in FIG. 2, the connection to the left steeringlever 42-2 is more fully illustrated to include a pull link 147 having afront clevis connection to a bottom pin 148 on the lever 42-2, a rearclevis having a connection to a top pin 150 on a bottom pivoted link152, and a pull link 154 connected to a mid-pin 156 on the link 152 andconnected to the left spool valve 44-2 so as to pull the latter againstthe resistance of spool valve return spring means 158.

STEERING HYDRAULICS-- FIG. 2

In the hydraulic circuit as shown according to this figure, near the topthe line 56 carrying line pressure splits up at 160 into a first branch162 connected to a composite pressure port 164 in the steering valveassembly 40 and into a second branch 168 connected to a compositepressure port 170. More specifically, fluid from the first branch 162 tothe port 164 flows in a path including a valve bore core 174 and aU-passage 176 connected to the port 164. Splitting off from that path,fluid also flows from the bore core 174, through the adjacent bore core178, thence through a passage 180 to a metering pressure port 166.

Fluid from the second branch 168 flows through a U-passage 184 to reachthe port 170, and also flows therethrough to a metering pressure port172 via a bore core 182, the adjacent bore core 186, and a passage 188thence to the port 172.

A first bore 190 of the consolidated valving assembly means 40 slidablyreceives the right clutch spool valve 44, and communicates with thepressure port 170 through a bore core 192, and with the pressure port172 through a bore core 194. The bore 190 communicates to drain D via abore core 196, a cross passage 195, and a drain line 198.

Through a bore core 202, the bore 190 supplies an inlet-outlet port 200having a right-steer lo clutch branch 204 therefrom leading to the loclutch cylinder 96C. The port 200 has a brake branch 206 supplying theright-steer brake cylinder 90B, via a bore core 208, a port 210, a borecore 212, a cross passage 214, a bore core 216, a port 217, a bore core218, a U-passage 219, and a brake conduit 220 leading to the brakecylinder 90B.

A third or brake bore 221 in the consolidated valving assembly means 40is connected to the line pressure connected right-steer branch 168 viathe bore core 182, a restriction 222, a U-passage 223, and bore coresfor the brake bore 221 shown at, respectively, 224 and 225 at oppositeends of the U-passage 223.

The inlet-outlet port 200 is controlled by a metering valve spool 226 inan end of the right-steer bore 190. The left-steer bore 190-2 has acorresponding, controlled inlet-outlet port 200-2, splitting up into afirst branch passage 227 hereinafter discussed and a second branchpassage 204-2 leading to the left-steer lo clutch 96C-2.

Between the left-steer bore 190-2 and brake bore 221, a brake passage228 conducts fluid from the former to the latter via a line 304, a borecore 229, and a bore core 230 which supplies the left brake line 220-2and the left brake cylinder 90B-2.

A common passage 231 connects the various chambers for return springmeans 140, 146, and 158 to drain D.

TOW VALVE-- FIGS. 2 AND 6

A slidable two spool valve 232 in the brake bore 221 has a telescopicfit with a brake metering spool 233 employed in the bore. Also employedin the bore are a land 234 and a land 235 which are on the brake spoolvalve 48 and separated by an intervening groove 236 on the spool.

A chamber 237 pressurizable with oil or grease during a towing emergencycontains therein the end of the tow spool valve 232 which presents apressure movable area 238. An emergency brake passage 239 communicatesthe grease directly to the right brake conduit 220 and brake cylinder90B and, via the spool groove 236, to the left steer brake conduit 220-2and brake cylinder 90B-2.

The grease is admitted to the system from the fitting of a gun, notshown, applied to a one-way nipple or grease valve 240, and reaches thebrake passage 239 through spool passages 241 and an interconnecting borecore 242.

TOWING-- FIGS. 5 AND 6

In a towing situation of the tractor, there is no line pressure and so,separately, external pressure must be introduced into the system.Pressure, of course, if present generally throughout the system wouldtend in an unwanted manner to apply the hydraulically applied clutch,and yet local pressure properly confined is necessary in order torelease the spring applied steering brakes of the vehicle.

The status existing for the emergency towing will be that the lines areunder no pressure and that a metering spring 243 against the meteringspool 233 is holding the two valve spool 232 in the position shown inFIG. 5.

Emergency fluid introduced under pressure through the one way valve 240,such as oil or the chassis grease pumped in by a hand lubricant gun,will be prevented by a drain-connected, pressure relief valve 245 frommaking escape and will be forced to flow through a path as shown in FIG.6, including the chamber 237, passages 241, bore core 242, brake passage239, conduit 220, and right brake cylinder 90B. The path of fluid flowfurther includes the brake passage 239, U-passage 219, spool groove 236,bore core 230, conduit 220-2 and left brake cylinder 90B-2.

In the valve position illustrated in FIG. 6 causing this pressurizedcondition, the three spool valves 232, 233, and 48 will be noted to beshifted in tandem to the right as illustrated. Therefore, tow valve 232intercommunicates passages 241 and the core 242, and the brake valve 48intercommunicates the bore cores 218 and 230, thus establishingcommunication between the brake cylinders 90B and 90B-2.

Accordingly, the right brake becomes released because pressurized, andthe depressurized lo clutch stays released to allow the vehicle to betowed.

Likewise, the second brake cylinder on the left side 90B-2 becomesreleased and the second lo clutch cylinder on the left side 96C-2 staysreleased.

Depressing the brake pedal will stop the vehicle under tow. The pedaloperated pull connection 138 will draw the valve 48 further to theright. Hence, the land 234 will uncover the U-passage 219 and dump bothbrake cylinders through the path to drain including bore core 225,U-passage 223, bore core 224, a bore core 316, and drain D.

ALTERNATIVE ACTION

The foregoing is the only exception to the general rule. The generalrule is that the clutch and clutch and brake on the right side of thetractor act solely in alternation to one another and the clutch andclutch and brake on the left side act solely in alternation to oneanother. For example, under low or moderately low pressure in bothhydraulic circuits of the system, the clutches will remain disengagedwhereas the brakes either will be fully engaged or will drag to theproper degree.

On the other hand under high or moderately high modulated hydraulicpressure, the clutches will individually be fully engaged or theaffected lo clutch will slip to the degree desired, while the brakeswill stay released.

SINGLE CONTROL PRESSURE

From the foregoing, and with specific reference to the lo clutchinlet-outlet port 200 of FIG. 2, it is evident that a single controlpressure applied to an inlet outlet port 200 will, depending upon itsvalue, set the tractor in selective conditions for, respectively, fullbraking, brake dragging, no clutching or braking, clutch slipping, andfull lo clutching without "fight" between a clutch on one hand and thebrake on the other. This inherent coordination makes for a significantsimplification of lo clutch control.

And in a simpler manner for hi clutching later to be described, thissame idea is carried forward for the hi clutching in alternation withbraking, similarly accomplished automatically without fight or doubledrive.

LIGHT/HEAVY SPRINGS-- FIG. 2

Typical for both sides of the tractor, the spring force of the brakesprings 92 for the right side is in the range of about 100-165 psi (7-11atmospheres) at the extremes of travel allowed the springs. For theclutch springs, the spring force at 98, for example, is in the range of50-100 psi (3-7 atmospheres) at the extremes of lo clutch spring travel.These figures are ones equivalent to, and in terms of, the actualhydraulic system pressure, and it will be seen that one set of springswill have completed its travel just when the other set of springs beginsits travel.

Thus if we increase the hydraulic pressure from 50 psi (3 atmospheres)to 165 psi (11 atmospheres), the lo clutch springs immediately begintheir contracting travel and complete this travel of contraction at 100psi (7 atmospheres) so as to bring the individual lo clutch discs intoinitial engagement, that is, just making contact but with no pressure ofengagement thereupon. Immediately after the 100 psi (7 atmospheres) isexceeded in this system, and contemporaneously with pressure ofengagement starting to be applied to the touching clutch discs, thebrake springs 92 begin collapse so that the brake discs, e.g., discs108B which are no longer under any residual spring contact pressure, areallowed to free themselves from contact and the disengagement continuesto 165 psi (11 atmospheres) when the brake springs reach their fullextent of contracting travel.

The reverse order is also true because, with decreasing circuitpressure, the brake springs 92 expand for the principal portion of theirtravel as hydraulic pressure drops from 165 psi (11 atmospheres) to 100psi (7 atmospheres). Then as the decreasing pressure continues droppingbelow 100 psi (7 atmospheres), pressure of contact building up in thebrake springs is exerted on the brake discs while the lo clutch discs110C, which are under no contact pressure, are free from the pressureplate 100 as the clutch springs 98 expand to the full extent of theirallowed travel. The lo clutch spring travel for total clutch release iscompleted at the point of a pressure drop to 50 psi (3 atmospheres) inthe hydraulic circuit.

Although we can doubtless attach significance to the separate andsuccessive sequencing of the spring sets because of inherently ensuringno clutch-clutch-brake "fight", there is some deeper significanceattached to the sequencing from coordinated spring rates, because ofinherent cushioning in hydraulically setting the brakes and engaging theclutches, now to be explained.

CUSHIONING OF CLUTCH-- FIG. 3

A desired pressure-time relationship is graphed in this figure leadingup to full clutch engagement, and an illustrative curve is showncomposed of connected linear segments and denoted by a, b, c, d, e, f,and g. The curve segment ab represents zero pressure maintained in thehydraulic circuit by the metering valve, not shown. When the meteringvalve is hydraulically shifted into its clutch-fill position, pressurerise is practically instantaneous as illustrated by the curve segment bcwhereas the segment cd denotes a slow rate of hydraulic pressure riseoccurring throughout the entire spring travel range of the clutchsprings, not shown. That is to say, spring travel of the clutch springstoward full spring compression means that the clutch cylinder, not shownis filling with progressively increasing volume and therefore readilyaccommodating the metering valve flow so that the pressure rises onlygradually. At the point d, the brake discs are in contact under nocontact pressure and the lo clutch discs are similarly in contact underno contact pressure.

Novelty is felt to reside in the situation represented by the curvesegment de. In that situation simultaneously with contact pressure onthe clutch discs being initiated and progressively increasing, the brakesprings, not shown, are undergoing their entire range of travel and animportant function is transpiring. The important function is that volumein the brake cylinder, not shown, is progressively enlarging and thusreadily consuming the metered fluid flow from the metering valve, thuslimiting the pressure rate of rise to the desired slow rate.Accordingly, clutch contact pressure rises linearly at slow controlledrate, rendering initial engagement of the clutch soft and free fromabrupt shock. Cushioned engagement of the discs means appreciablyreduced wear on their friction engageable surfaces.

The operation represented by the curve segment ef is final pressureapplication to the lo clutch discs which follows at a fast rate of risein pressure for complete clutch engagement; practically no clutch slipis allowed during this operation. The completed engagement isrepresented by the curve segment fg, with the lo steering clutchinvolved being fully engaged for a fixed 1:1.3 step-down gear ratio atone side of the vehicle and the companion brake being fully disengagedso as not to interfere.

CUSHIONING OF BRAKE-- FIG. 4

A pressure-time relationship is graphed in this figure illustrative ofthe events culminating in full spring-applied brake engagement, and theillustrative curve is composed of interconnected linear segments and isdenoted h, i, j, k, l, m, and n. The curve segment hi represents thehydraulic system at one side of the vehicle being under full modulatedpressure as directed thereto by the metering valve on that side, notshown. Shift of the metering valve into a position allowing meteredescape of the hydraulic fluid from the system is represented by thecurve segment ij, illustrating a somewhat instantaneous or abruptdecrease of pressure in the system. Full travel of the brake springs,not shown, due to their selected spring rate, is represented by thecurve segment jk. That is to say, the brake springs expand over theirprincipal range of travel and the effective volume of the brake cylinderprogressively decreases. The effect of the emptying brake cylinder isthat the rate of pressure drop is comparatively slowed down at theoutflow from the cylinder makes its way through the metering valve, notshown. And upon reaching the point k on the curve, the condition asreflected at that point dictates that the brake is in the status ofhaving a fully collapsed cylinder and the brake discs are in contact butunder no pressure, whereas the lo clutch cylinder still has the statusof being full of oil but the clutch discs, not shown, are in contactunder no contact pressure.

Novelty is felt to be present throughout that operation represented bythe segment kl of the curve Throughout that operation, decreasingcylinder pressure in the fully extended lo clutch cylinder, not shown,allows the clutch springs, not shown, to collapse the cylinder throughtheir entire range of travel represented by the curve segment kl. Butescaping fluid from the emptying clutch cylinder requires an interval tomake its way through the metering valve, not shown, and so the rate ofpressure decrease is reduced during this initial application of thespring applied brake. Brake disk pressure is therefore slowly applied bythe brake springs and a soft gradual brake engagement interval ensues.Cushioned engagement of the disks provides significantly increased lifefor their friction engageable brake surfaces.

The curve segment lm represents the brake spring action followingcollapse of the clutch cylinder. So the final cylinder pressurereduction along the segment lm is at a rapid rate and brings on thebrake application at full brake spring pressure. Thereafter, theunopposed, full-brake application interval is represented by the segmentmn on the pressure-time curve. The brake is therefore fully spring-setat the vehicle side concerned, and the companion lo clutch is fullydisengaged so as not to interfere.

MODULATION-- FIGS. 3 AND 4

Between the pressurization condition of full clutch-and-brake pressure,which obtains in the system when maintaining full lo clutching asfinally reached in FIG. 3, and the pressurization condition of 0 drainpressure, which obtains in the hydraulic system when maintaining fullbraking as finally reached in FIG. 4, there lies an infinity ofmodulated pressures falling in the intermediate range. The steeringpressure, right, and the steering pressure, left, in the tractor areindependent of one another and, irrespective of what exists on the leftside, for example, the steering pressure on the right side controllingspeed of the right drive sprocket can be fixed at any point in theintermediate range, or in either extreme of pressurization, or can beundergoing a raising or lowering of pressures either is some sequencewith one another or as an individual instance.

For instance, during lo clutching as shown being accomplished in FIG. 3,the set pressure in the lo, friction engaging, drive device concernedcan be fixed at the level u which can be seen to intersect the clutchengaging curve at a low pressure point corresponding to a small amountof torque being transmitted and considerable lo clutch slippage. Or, ifpressure is set at the level v which intersects the clutching curve atthe point e thereon, the pressure will be fixed to afford thetransmission of a moderate amount of torque and moderate slippage in thelo friction engaging drive device concerned. At the set level w at thepoint there the clutch engagement curve is intersected, higher torquewill be transmitted and small slippage will occur in the frictionengaging lo drive device concerned.

Similarly with braking in accordance with the braking curve illustratedin FIG. 4, brake force can be set at the level x which intersects thebraking curve at a point corresponding to a lightly dragging brake. Orat the set level y which intersects the braking curve of FIG. 4 at pointl, there will be moderate brake drag because of the increasinglyunopposed brake spring pressure in the friction engaging brake device.At the level z where it intersects the braking curve, the frictionengaging brake device will be forcefully arresting motion because of thecondition of the brake springs approaching being unopposed at that pointby fluid pressure.

According to the invention the set values just considered and other suchvalues, or the changing of steering pressure is controlled with greatprecision and exactitude by the means which I provide for that purpose,as will now be explained.

RIGHT SPOOL VALVE 44-- FIG. 5

Among other spools interspersed along the length thereof, theright-steer valve 44 appearing in this figure carries a perch spool 244,a narrow blocking spool 246 having a narrow groove 248 thereadjacent,and a control spool 250. The valve 44 also: carries, distally, a drainspool 252 adjacent the control spool 250; carries, proximately, a hiclutch spool 254 adjacent the perch 244; and, separated by a valvegroove 256 from the high clutch spool 254 and separated from the drainspool 252 by an annular recess 258 and by the spool and groove 246 and248, the valve 44 carries a pressure spool 260. To varying degreesdepending upon the longitudinal position to the right into which thespool valve 44 slides in the right bore 190, the exterior of the drainspool 252 variously communicates with a bore groove 261 which is atdrain pressure and so, with like communication, does a helix 262 whichis grooved into the spool exterior of the pressure spool 260communicates variously with line pressure from the annular groove 256which is supplied by the composite pressure port 170 previouslydescribed.

The helices 262 and 264 have uniform groove depth, uniform groove size,and uniform helix angle so that the resulting orifice which each helixforms with the closing surface of the valve bore 190 has constant rateof pressure drop along its length after the standard manner of pressuregradient of accurate hydropotentiometers. The resulting small quantitiesof oil flow are similar to the small flows of electric current in anelectric potentiometer (not rheostat).

A signal pickup port 266 formed in the valve 44 in the base of theannular recess 258 communicates through a relatively small radialpassage drilled in the valve and through an interconnecting relativelylarge longitudinal passage 268 with an intervalve chamber 270 defined inthe bore 190 between the metering spool 226 and an end spool 269 of theright steer spool valve 44. In its communicating function in the solidline position of the valve 44 as shown in solid lines in FIG. 5, thesignal pickup port 266 will cause the intervalve chamber 270 to bepressurized at signal pressure being introduced by the compositepressure port 170 and by the valve spool annular groove 256. The orificedefined by the helix 262 on the drain spool 252 will be subjected to thefull pressure drop from full 290 psi line pressure in the recess 256 tothe drain pressure which is maintained in the bore groove 261 by reasonof the latter groove's connection 271 to drain D.

By reason of the resulting physical invention of the pressure spool 260,a light outward pull exerted on the spool valve 44, shifting it in adirection slightly to the right from the position as shown in FIG. 5,will cause the annular recess 258 to be isolated from the full linepressure of the bore groove 192, and a portion of the orifice defined bythe helix 264 will become active in supplying the signal pickup port 266with pressure of slightly reduced value. At the same time, a furtherportion of the orifice defined by the helix 262 will become active asthe helix withdraws from bore groove 261, rendering the pickup port 266definitely at an intermediate point in the overall effective orificelength. Further pressure reducing shift of the spool valve 44 to theright will cause the signal pickup port 266 to slide to a position morenearly approaching the drain pressure and therefore supplying theintervalve chamber 270 with a like reduced static pressure.

Finally, shift of the spool valve 44 to its extreme of rightward travelas viewed in FIG. 6 will block off the recess 258 and groove 192 andwill place the signal pickup port 266 exclusively at drain pressure,thereby establishing the drain pressure in the intervalve chamber 270.In this manner, the pressure of intervalve chamber 270 can be accuratelyfixed or accurately varied among first, signal pressure, second drainpressure, and third an infinity of accurately-held pressures in anintermediate range between maximum and minimum as defined by signal anddrain.

RIGHT METERING SPOOL-- FIG. 6

The metering spool 226 as illustrated in this figure has a centersection of reduced diameter which defines a bore passage 272 and whichintegrally interconnects a solid land 274 and a hollow land 276 of thespool 226. An actual-pressure pickup port 278 in the reduced centersection communicates externally through the bore passage 272 and thebore groove 202 with the right inlet-outlet port 200, and communicatesinternally through a radial passage and a longitudinal passage 280 inthe hollow land 276 with a spring chamber 282 which is at an oppositeend of the spool 226 from the intervalve chamber 270, previouslydescribed.

Adjacent the reduced diameter center section of the spool 226, the solidland 274 has a medially disposed cylindrically stepped relief 284 on theshoulder; in an extreme position of the metering spool 226, as moved inthe direction to the right as viewed in FIG. 6, the relief 284 willcommunicate with the bore groove 196 which is at drain pressure, and theactual-pressure pickup port 278 and bore passage 272 will thereforecommunicate drain pressure to the spring chamber 282 and to theinlet-outlet port 200. The opposite extreme position will now bedescribed.

Adjacent its juncture with the reduced diameter center section of thespool 226, the hollow land 276 has a medially disposed cylindricallystepped relief 286 at the shoulder. In the solid line position of thespool 226 as shown in solid lines in FIG. 6, the relief 286 is out ofcommunication with line pressure; however, the relief 286 willprogressively provide communication with line pressure as the meteringspool 226 shifts leftwardly as viewed in FIG. 6. Thus a metered amountof line pressure up to full line pressure can be communicated from themetering pressure port 172 through the relief 286 for ultimatecommunication with the inlet-outlet port 200 and the spring chamber 282at one end of the spool 226.

A weak spring 288 which is in the hollow interior of the land 276 andwhich seats on an end plate 290 of the steering valve assembly 40,lightly biases the spool 226 in the direction of the opposing muchstronger valve return spring set 146; the weak spring 288 serves, amongother things, to prevent the metering spool 226 from drifting if, at atime when it is depressurized at 282, it happens to be out of physicalcontact with the end spool 269 of the right spool valve 44. In onephysically constructed embodiment of the invention, the weak spring 288had an equivalent force of 20 psi (1.4 atmospheres). So the springchamber 282's pressure, which is the same as maintained at theclutch-and-brake inlet-outlet port 200, is established always at 20 psiless than signal pressure as maintained in the intervalve chamber 270.

SIGNALING-METERING-- FIG. 6

The first bore 190 and the spool elements therein respectively: have therestrictive right branch 168 connecting high pressure thereto from themain pressure line 56, through split 160 and said branch 168 into thebore core 182; have the same branch 168 connecting the high pressurethereto through the U-passage 184 into the composite pressure port 170and bore groove 192; have the same branch l68 connecting the highpressure thereto through the bore core 182, a spool groove 291, theadjacent bore core 186, and the passage 188 into the bore groove 202;have a conduit 292 connecting the low pressure of drain thereto leadingfrom the bore groove 196; have the low clutch line 204 connecting theinlet-outlet pressure thereto from the low clutch 96C and said line 204into a side passage 293, inlet outlet port 200, and bore groove 202; andhave the high clutch passage 132 connecting the inlet-outlet pressurethereto from the high clutch 97H and passage 132 through a bore core 294and port 295 into the bore groove supplied by the composite pressureport 170.

By controllably connecting two of the high pressure and low pressureconduits just described, the pressure and drain spools 260 and 252 ofthe right spool valve 44 precisely set the signal pressure at a desiredvalue in the intervalve chamber 270, and the metering spool 226 at itsend adjacent the intervalve chamber 270 is exposed to that pressurebecause of the latter's communication thereto in the valve 44 throughthe signal pickup port 266 and longitudinal passage 268. As alreadyindicated, the signal pressure established by the valve 44 variesbetween the high and low pressure conduit from a maximum to a minimum;that variance is proportionally in accordance with the effective lengthof orifice defined by the helices 264 and 262 on the spools 260 and 252.

The control orifices formed in the metering spool 226 by the reliefs 286and 284 vary in size and effective fluid handling capacity dependingupon how much each orifice projects at the end outwardly beyond itsadjacent end of the bore passage 272. Thus, each particular longitudinalposition of the spool 226 establishes the relative rate of inflow ofpressure fluid and outflow of drain fluid through the ends of the borepassage 272, accordingly establishing the pressure of the inlet-outletport 200 between the high and low pressure conduits from a maximum to aminimum.

In theory, the metering spool 226 stabilizes in its position at thepoint when inflow through the relief 286 into passage 272 and theoutflow therefrom through the relief 284 equalize. At that point, if weassume zero clutch leakage, the pressure in the spring chamber 282 whichis at one end of the spool 226 and which equals inlet-outlet pressure,is also essentially equal to the signal pressure in the intervalvechamber 270. Or, more generally, actual inlet-outlet pressure in thechamber 282 equals desired pressure set in the chamber 270, if the minorbias of the weak spring 288 can be ignored, which it can be for myprincipal considerations.

In actual practice, however, the valve cracks the relief 286 open enoughalways to compensate for clutch leakage flow and usually, if not always,the drain-connected relief 284 will stay closed except during clutch orbrake evacuation.

It is primarily, of course, the strategic location and arrangement ofpickup ports 278 and 266 and longitudinal spool passages 280 and 268 inthe valve elements which makes it possible to have these elementscompactly arranged so as to fit end-to-end in the bore 190 whichslidably receives them in common. These valve elements are totallyunlike in function as they mutually relatively move in their bore, andthey have a strict master-follow-up relationship. The right valve 44 isthe element serving as the exact-static-pressure-dictator andposition-dictator, and the spool 226 serves as automatic follow-upelement to take a corresponding satisfied, dictated position in the bore190.

FULL LO CLUTCHING ON RIGHT-- FIG. 5

Similarly occupying the full clutching position as shown for the leftspool 44-2, the right spool valve 44 is shown in this figure in aposition supplying line pressure to the right clutch cylinder 96C and,likewise to the right brake cylinder 90B. In other words, main pressurefrom the line 56 leads in a path through the right restriction branch168, the bore core 182, the spool groove 291, a bore core 186, a passage188, the metering pressure port 172, a bore groove 202, thence into theinlet-outlet port 200 where it splits in branches flowing one waythrough the lo clutch line 204 into the lo clutch cylinder 96C, andflowing the other way through the passage 206, the bore core 20B, aspool groove 297, the bore core 212, the cross passage 214, the borecore 216, the port 217, the U passage 219, thence through the brakeconduit 220 into the right brake cylinder 90B.

At the same time, the solid land 274 of the metering spool 226 blocksthe right end of the valve passage 272, preventing fluid in the inletoutlet port 200 from escaping from the relief 284 and bore core 196 todrain D through drain line 198.

In the desired way, therefore, the right clutch cylinder 96C will forcethe lo right clutch disks, not shown, into full engagement and the rightbrake cylinder 90B will force the right brake piston, not shown, intofully disengaged position.

FULL BRAKING RIGHT VALVE 44-- FIG. 6

For braking at the right side, the spool valve 44 from its valve-inposition is pulled by the lever connection 144 to its full valve-outposition as shown in solid lines in FIG. 6. The metering spool 226follows the valve 44 part way under bias of the light spring 288 in thespring chamber 282, thus occupying its full rightward positon as shownin solid lines in FIG. 6. Because the signal pickup port 266 of thevalve 44 is connected by the helical groove 262 with the drain D throughthe drain line 271, the pickup port 266 is at drain pressure andcommunicates the drain pressure through the valve longitudinal passage268 to the intervalve chamber 270. The metering relief 286 of themetering spool 226 is covered, blocking off the inlet-outlet port 200from line pressure, whereas the control orifice formed by the relief 284and the bore passage 276 establish the inlet-outlet port 200 at thedrain pressure of bore groove 196. Hence, the actual-pressure pickupport 278 communicates drain pressure through the longitudinal passage28O and hollow interior of the hollow land 276 to the spring chamber282.

Pressures at opposite ends of the spool 226 are thus equalized. Hence,the right brake and right lo clutch cylinders, not shown, empty in thatsequence through the respective brake conduit 220 and clutch conduit 204so that the right brake 90B is fully spring engaged and the right clutchpressure plate, not shown, in the lo clutch 96C is fully disengaged.

DRAG: ON BRAKE CAUSED BY VALVE 44-- FIG. 7

In the illustrated position of the right-steer spool valve 44 in thisfigure, in which the corresponding system pressure will be 40 or 60 psi(three atmospheres or four atmospheres), for example, the valve 44 willcause partial braking at 90B on the right side, such as for a steeringturn. In order to do so, the signal pickup port 266 in its relation tothe settings of the respective double helices 264 and 262 will firstcommunicate the desired signal pressure to the intervalve chamber 270.

The metering spool 226 is shown making its resulting, final slightadjustment in the rightward direction of the arrow 300 into fullybalanced or satisfied position. And, when finally adjusted into thesatisfied position, the stepped relief 286 forming the control orificeof the spool 226 will be barely uncovered to allow fluid flow at a slowrate under high pressure drop, whereas the spool's stepped relief 284will be slightly more uncovered and allow slow flow at the same rate butunder a smaller pressure drop leading thru line 198 into drain D. Thechambers 282 and 270 under the respective actual and desired pressuresat opposite ends of the spool 226 will be essentially equalized inpressure.

SLIP CLUTCH LO DRIVE ON RIGHT-- FIG. 8

The right-steer spool valve 44 is shown adjusted in, and the meteringspool 226 is shown stabilized in, the position for partial right side loclutching corresponding to an hydraulic pressure of 120 or 150 psi(eight or ten atmospheres), for example, in the system. In such valveposition, the pickup port 266 for the right-steer valve 44 will becommunicating the desired pressure to the end of the metering spool 226confronting the intervalve chamber 270. An essentially equal andopposite pressure will be communicated by the pickup port 278 into thespring chamber 282 at the end of the spool 226 adjacent the valve plate290. A minor, steady flow of fluid from the pressure port 172 to thebore groove 196 and line 198 to drain D will be maintained by themetering spool 226 after it stabilizes, so that the orifice through thestepped relief 286 provides a relatively small pressure drop as thevolume of flow makes entry into the bore passage 272, whereas when oneconsiders theory and ignores clutch leakage the control orifice throughthe stepped relief 284 causes a relatively large pressure drop with thesame volume of flow in passing from the passage 272 to drain.

The resulting slipping clutch lo drive at the right side of the tractorwill, if complemented by a full clutching lo drive onthe left side, notshown, produce sweeping turns or else more gradual steering turns to theright as desired. Such complementing lo drive will now be described.

OPERATION: SECOND CLUTCH-CLUTCH-BRAKE VALVE-- FIG. 5

The left or second clutch-clutch-brake spool valve 44-2 and meteringspool 226-2 are illustrated in this figure in the full clutching loposition. The accessibility of the second bore 190-2 compared to thefirst one 190 is essentially the same: for pressure from the compositepressure port and metering pressure port 164 and 166; for drain fromdrain bore groove 296 thence to drain line 271; for drain from the drainbore groove 298 thence to the drain line 198; for inlet-outlet pressurefrom inlet-outlet 200-2 thence to the lo clutch line left 204-2; forinlet-outlet pressure in the branch leading from passage 227, throughbore core 300, bore core 302 and conduit 228, thence into conduit 304,bore cores 229 and 230, and left brake conduit 220-2; and forinlet-outlet pressure from bore core 305 thence into hi clutch conduitright 306-2 for the hi clutch cylinder 97H-2.

For the sake of brevity, a disclosure is omitted of the variouspositions of the valve elements in the second bore 190-2 for causing noclutching or braking, partial clutching, partial braking, and fullclutching and full braking on the left side of the tractor.

FOOT BRAKE SPOOL VALVE 48-- FIG. 5

In reference to the brake bore 221 shown receiving it in this figure,the foot brake spool valve 48 is shown positioned all the way on,producing no braking. Consistent with its purpose in that position, andfor controlling the right side of the tractor, the foot brake valve 48provides an unimpeded braking connection between the cross passage 214from the right steering valve 44 and the right brake cylinder 90B, in apath including the passage 214, the bore core 216, the brake bore port217, the core and U-passage 218, 219, and the right brake conduit 220.Therefore, the right steering valve 44 modulates pressure in the brakecylinder 90B as desired, and the foot brake valve 48 does not interfere.

Similarly, the left steering valve 44-2 modulates the pressure in theleft brake cylinder 90B-2 unimpededly, in a path including bore core200-2, passage 227, bore core 300, bore core 302, conduit 228, conduit304, bore core 229, bore core 230, and brake line 220-2 leading to theright brake 90B-2. Hence, the steering valves 44, 44-2 can hold bothsteering brakes released, and the brake valve 48 introduces braking onneither side of the tractor. However, by modulating the pressures in thebrake cylinders, which the brake valve 48 can do by progressivelydumping them to drain, the brake valve can cause the brakes to be setfor various degrees of drag or be fully set as will now be explained.

DRAG: ON BRAKE CAUSED BY VALVE 48-- FIG. 7

When, by means of the pedal connection 138, the brake spool valve 48 ispulled, the initial activating movement enables the brake land 234 asillustrated to block off the bore port 217 leading to the right brakeline 220 and the right brake cylinder 90B. Similarly, the initialmovement allows the brake valve land 235 to block off the bore core 230leading through the left brake line 220-2 to the left brake cylinder90B-2.

However, in order to ensure brake cylinder dump paths responsive tosteering valves 44 and 44-2 following the noted initial movement of thebrake valve 48, alternate dump paths are available leading to drainfirst, from the brake cylinder 90B, brake line 220, brake passage 239,bore core 242, brake bore 221, check valve line 308, bore core 216,cross passage 214, bore core 212, port 210, bore core 208, passage 206,and inlet outlet port 200 leading thru line 198 to drain D; and second,from the left brake cylinder 90B-2 through the brake line 220-2, conduit310, brake bore 221, check valve line 312, and line 228 controlled bythe inlet outlet port 200-2 which is connected to drain appropriatelythru the line 198 by the left steering valve 44-2.

The valves in the check valve lines 308 and 312 are ball check valveswhich unseat in the direction of the steering valves and drain; thusthey do not interfere with the individual brake cylinder dump paths andtheir function is to prevent cross flow between those two paths.

From the position as shown in solid lines in FIG. 7, in which the brakevalve 48 has temporarily isolated the brake cylinders from modulation bysuch brake valve 48, further pull out of the valve 48 to the right, asviewed, causes a progression of events in sequence. First, in progressof outward movement of the valve 48, the right and left brake cylinders90B, 90B-2 are intercommunicated by the intervening groove 236 of valve48 for coordinated application of braking on both sides of the vehicle.Next, in pull out progress of the valve 48, the brake valve land 234directs pressure fluid from the U-passage 223 thru the U-passage 219 toboth brakes 90B and 90B-2. The line pressure conduit 56 feeds suchpressure fluid to the U-passage 223 in a path including junction 160,restriction 168, bore core 182, restriction 222, and U-passage 223,which latter would tend to have high pressure except for the interveningdumping function of the brake metering valve spool 233. At this pointthe longitudinal load on a metering spool spring 314 is equivalent toabout 75 psi (5 atmospheres;, and so the metering spool 233 regulatesthe pressure in U-passage 223 to about 75 psi by dumping through a pathincluding the brake bore core 316 leading to drain D. The resistance ofa constantly acting or full travel return spring 318 on valve 48 startsto be supplemented by a strong, lost motion travel, brake valve returnspring 320 to indicate brake application to the operator. To make up forbrake cylinder leakage, minor fluid flow thereto coming from therestriction 222 is maintained at the 75 psi level so that there is rightdrag on both brakes.

Foot braking has no effect on the clutch engagement at either side ofthe tractor. So if the steering valves have the respective positions forpartial clutching lo on the right side and full clutching lo on the leftside (FIG. 8), the left and right tracks will have unequal drive on themat the two sides of the tractor, which therefore will execute at veryslow speed a right turn, slow in the respect that both tracks are beingpartially braked by action of the foot brake. It is evident thegraduality or sharpness of the resulting turn will depend in part on theterrain, e.g., clay surface, gravel surface, and so forth.

Next in pull out progress of the brake valve 48 in FIG. 7, thelongitudinal compression on the spring 314 becomes more and more relaxedand the cylinder brake pressure proportionally reduces for increased,equalized braking at both sides.

This hydraulic braking is proportional to the pedal travel, up to andincluding full braking now to be described.

FULL BRAKING, FOOT BRAKE VALVE 48 -- FIG. 8

When the brake spool valve 48 is pulled by means of the brake connection138 and the foot brake and into the full brake position as illustratedin this figure, the intervening groove 236 equalizes the hydraulicpressure between the right and left, spring-applied brake cylinders 90Band 90B-2. By a common path the two cylinders are jointly connected todrain through a passage 219, bore core 225, U-passage 223, bore core224, thence brake bore core 316 to drain D.

Hence, unopposed equalized full brake spring pressure is applied to bothsides and the tractor is braked to a full stop. The restriction 222offers substantial resistance preventing the drain path from undulylowering line pressure and from affecting independent operation of thesteering clutches while the foot brake is operating.

DETENTED LO TO BR OPERATION -- FIGS. 2 AND 5

Each steering lever is hydraulically detented in mid range of travel ata partially retracted LO position, as indicated in solid lines for theleft lever 42-2 in this figure of drawing. Both valves 44 and 44-2 havethat hydraulic position as shown in FIG. 5 and are each held inequilibrium there.

More specifically in the example of the hydraulically detented valve 44in FIG. 5, the right clutch pressure from the restriction 168 istransmitted directly to the intervalve chamber 270 via the U-passage184, port 170, spool groove 256, a bore core or pressure chamber 321 anda pick up port 266, and thence through passage 268 to said chamber 270.The outwardly directed force moves until counterbalanced in bothdirections in the bore 190.

First, the metering spool valve 226 moves to the left, to a point atwhich the increasing metered output pressure exerted in end chamber 282creates a force which, when supplemented by the increasing force of thecollapsing valve spring 288, balances the equal opposing force frompressure in the intervalve chamber 270. Second, the steering spool valve44 moves to the right, as viewed in FIG. 5, until a so-called second,lost-motion travel spring 322 in the spring assembly 146 is engaged and,in a so-called second position, the spring assembly mechanicallybalances the equal opposing force from pressure in the intervalvechamber 270. In the third position of the valve and spring assembly 146,two other springs namely, a first full travel spring 324 and anotherlost motion travel spring 326, are active and, from his hand on theright steering lever, the operator will have the feel from the controlthat he has reached a static, detented valve position.

In this resulting detented or second position of both valves 44 and 44-2as shown in solid lines in FIG. 5: flow is minimal; each metering returnspring 288 which has an equivalent force of 20 psi (1.4 atmospheres)keeps lo clutch pressure 20 psi below the full line pressure existing inline 56; the valve parts are in static balance; the lo clutches arefully engaged; and the steering brakes (also hi clutches) are fullyreleased.

In operation of the steering levers when they both occupy the full loclutch position in which the left lever 42-2 is shown in solid lines inFIG. 2, the lever 42 or 42-2 at either side of the tractor can begradually rearwardly tilted by the operator from LO toward BR as tocause the track at that side to become, respectively, only partiallyclutched, only partially braked, or full braked causing a sweeping,gradual, or abrupt turn of the tractor to that side.

Or viewed the other way, in tilting the steering lever graduallyforwardly from BR, the operator causes the steering pressure at thatside to go from zero to 100 psi (7 atmospheres) in the exampleillustrated, changing the steering brake disks from full contactpressure to release, or zero contact pressure. The steering clutch isdisengaged and remains disengaged to 100 psi (7 atmospheres).

Then during increase from 100 psi to 270 psi (7 atmospheres to 18atmospheres) of internal hydraulic pressure, the pressure forcing thedisk plates of the clutch together at the side increases from zerocontact pressure to full clutching pressure.

While the tractor can be braked to a full straight stop by pulling thesteering levers 42 and 42-2 to the rear simultaneously, the simple wayintended for producing a full straight stop is by full depression of thefoot brake 46. In one physically constructed embodiment of the inventionthe steering brakes, under their full stop setting, could be overcomewith engine torque only with the transmission gearing set in the lowestgear and with full power clutching.

As herein disclosed the lo clutch release springs 98, which can becharacterized as light springs, are described as allowing the clutchpiston at 50 psi (3 atmospheres) to start moving and then at 100 psi (7atmospheres) to encounter the solidness of the pressed-together clutchdisks which the piston contacts. The 100 psi demarcation point for fulllo clutch release and onset of brake engagement is made manifest to thefeel of the operator, and the left steer spool valve 44-2 has thecorrect position as shown in solid lines in FIG. 8 to illustrate thesituation. Specifically, a third, lost motion travel spring 328 comesinto engagement at the 100 psi demarcation point. The operator feels theextra resistance build-up as a rise in steering lever effort when thevalve 44-2 under pull at 154 first encounters the third spring 328 ofits spring assembly 158.

It is evident that, if desired, the lo clutch release springs can bemade somewhat weaker, allowing the clutch disk plates to be in contactwith the clutch piston before the brake can have reached 100 psi (7atmospheres) and have fully released. Or if the brake springs 92, whichcan be characterized as heavy, are made somewhat stronger, the sameeffect can be made to occur whereby the lo clutch piston will have comeinto contact with and started compressing the lo clutch plates beforethe brake is fully released. In these ways the overlap, whereby both thelo clutch and the brake are slightly engaged at the same time on one orboth sides of the tractor, can be varied to suit individualcircumstances and needs.

290 PSI 1ST SIGNAL FOR F C HI -- FIG. 6

A 20 atmosphere or 290 psi line pressure signal which is supplied by asteering valve in its first main drive home position of full retractionF C HI will operate the steering mechanism into its full hi clutchingsetting from any other setting, and the corresponding full hi clutchsetting of the left steering lever appears in the broken line positionmarked HI in FIG. 2.

The corresponding full hi clutch setting of the left steering valve 44-2appears in solid lines in FIG. 6. Fluid under line pressure enters thevalve 44-2 through the restriction 162, and flows on through in firstand second paths until it fills the left hi clutch cylinder 97H-2 and,therefollowing, fills the left brake cylinder 90B-2 and then appliesfull pressure so as to clutch and unbrake, respectively. The first oneof the paths, which is to 97H-2, is via 174, 176, 164, 305, and 306-2.The second path, which is to 90B-2, leads there via 174, 302, 228, 229,235 (open position), 230, and 220-2.

In order to achieve clutch application cushioning effects of the naturepreviously described in connection with the FIG. 3 clutch embodiment,each set of hi clutch release springs such as a set 330 for hi clutchpiston 95H in FIG. 2, has a 60 psi-100 psi (4-7 atmospheres) equivalentpressure range; so 60 psi pressure in the clutch cylinder 97H is equalto the spring force and the cylinder starts filling and at 100 psipressure the springs are compressed at full travel, with the cylinderfilled but without exerting contact pressure on the disks lllH.

Then I can achieve full clutching pressure by a further increase wherebythe cylinder 90B of the spring applied brake introduces (between 100 and165 psi) full take-up travel afforded on the brake disks 108B whilefilling, and while at the same time the clutch disks lllH are beingengaged with a cushioning action due to the delay time required byliquid through the common port 174, FIG. 6, needed to meet the liquiddemand in filling the brake cylinder 90B.

CLUTCH-CLUTCH TRANSITION AT NEUTRAL POINT -- FIG. 7

At the neutral transition position Nl as shown in solid lines in thisfigure for the left steer valve 44-2, no first signal for the left hiclutch 97H-2 is being developed because the valve 44-2 blocks the clutchconnected port 164, and no second signal for the left lo clutch 96C-2 isbeing developed for the same reason because valve 44-2 blocks port 164.However, the bore core 302 keeps the left brake 90B-2 disengaged bytransmitting disengagement pressure thereto from branch 162 through thebrake-connected passage 228.

The operator realizes by feel that he is making a transition betweenclutches because of increased spring resistance occurring in position Nlwhen a lost motion travel intermediate spring 326-2 is engaged. In otherwords, the constant bias of the first spring 324-2 is immediatelyaugmented by bias from spring 326-2.

Aside from springs 324-2 and 326-2, the operator encounters nothingfurther in the way of valve resistance in the receding direction inmoving it into and beyond the neutral position N1 toward the full loclutch position of the valve 44-2 as illustrated in FIG. 5. Althoughvalve 44 in FIG. 7 is not illustrated with a vacuum breaker valve showntherein, it is well within the skill of the art in the vicinity ofintervalve chamber 270 to equip the end of valve 44 between groove 297and passage 268 with an interconnecting internal vacuum breaker checkvalve for using the bore liquid from the clutch-brake core 208 toreplenish the liquid in chamber 270 as the receding valve 44 enlargesit.

270 PSI SECOND POSITION FOR F C LO -- FIG. 5

This figure illustrates both steer valves 44 and 44-2 providing full loclutching and driving both tractor tracks under identical lo positivedrive speeds. The hi clutches 97H and 97H2 are shown blocked fromreceiving a pressure signal and connected to the common drain line 231and drain D in FIG. 5.

The brakes 90B and 90B-2 are fully disengaged.

270-100 PSI 2D SIGNAL LO CL. RANGE -- FIGS. 4, 8

This range appears graphically in the lower half of FIG. 4, and theright steer valve 44 is illustrated in solid lines positioned at anintermediate point in that range in FIG. 8. The sequence, as theoperator withdraws the valve 44 and enlarges the intervalve chamber 270,is to go from full lo clutching pressure at 270 psi (18 atmospheres),through some intermediate clutching pressure producing partial loclutching with appropriate clutch slip which could be attained in thevalve 44 solid line position of FIG. 8, to a neutral position at 100 psi(7 atmospheres) pressure in which there is no disk contact pressure ineither the lo clutch or the brake.

CLUTCH-BRAKE TRANSITION AT NEUTRAL POINT -- FIG. 8

At the neutral transition position N2 as shown in solid lines in thisfigure for the left steer valve, the operator realizes by feel that heis making a transition releasing clutching and engaging braking becauseof increased spring resistance occurring when a lost motion travel thirdspring 328 is engaged by the valve 44-2. In other words, the collectivereturn spring bias of the valve's first spring 324-2, intermediatespring 826-2, and second spring 322-2 is immediately augmented by biasfrom spring 328 to prevent overbraking starting at the transition point.

100-0 PSI 3D SIGNAL BR RANGE -- FIGS. 4, 7

This range appears graphically in the upper half of FIG. 4, and theright steer valve is illustrated in solid lines positioned at anintermediate point in that range in FIG. 7. The sequence, as theoperator withdraws the valve 44 and enlarges the intermediate chamber270, is to go from a transition position at 100 psi (7 atmospheres)pressure in which there is no disk contact pressure in the brake (or inthe lo clutch), through some intermediate braking pressure producingproportional partial braking with appropriate brake dragging which couldbe attained in the valve 44 solid line position of FIG. 7, to a fullspring-applied disk braking pressure occurring under zero hydraulicpressure.

A direct change in the steering mechanism from its full brake setting toa full hi clutch setting (FIG. 6 showing of valve 44-2) is, as noted,attended by cushioning of the clutch engagement by reason of theaccumulator effect of the brake cylinder as it is required to be filledthrough the common port to core 174.

Vice versa however the independent passages 231 and 292 provide separatehi clutch and brake cylinder emptying paths to drain D, so that a directchange from full hi clutch setting to full brake setting (FIG. 6 showingof right steer valve 44) affords no cushioning of the brake engagementby reason of the emptying of the right hi clutch cylinder 97H. What thischange of setting amounts to in effect is the direct application of theunmodulated third signal, so-called, (0 psi) to the hi clutch and brake,in concert. So their engagement action is still one of inherentalternation to each other but with cushioning, as between the two,available during the hi clutch engagement only.

LO SEEKING AUTOMATIC RESET -- FIGS. 2, 5

When the operator releases a steering lever, such as the left lever whenin the broken line braked position as shown in broken lines BR in FIG.2, the return spring assembly, not shown, causes the left steer valveconcerned to seek out the lo clutch position so that the lever 42-2moves only part way, and resets in and is hydraulically detented in theLo position as shown in solid lines in FIG. 2. The reason is that, inthe full lo clutch position of the right steer valve 44 as shown in FIG.5, the three return springs first 324, intermediate 326, and second 322expand only until they reach the position shown in which their combinedthrust exactly balances the opposing hydraulic force of signal pressurein the chamber 270 which has reached 290 psi (20 atmospheres) inpressure.

No hydraulic lock can form, interfering with valve 44's motion to ahydraulically detented stop. The bore core 321 acts as a low pressure,back pressure chamber into which the spool passage 268 can afford anescape path for the collapsing intervalve chamber 270's fluid. Themotion is arrested, preventing valve overtravel, because the passage268's pick-up port 266 as it leads into the back pressure chamber 321forms a restriction to flow in the escape path. Because the spring forceof the second spring 322 is hydraulically countered by the intervalvechamber pressure, the converse is true that the spring 322 is active insubstantially augmenting resistance to prevent the valve 44 fromwithdrawing to enlarge chamber 270 and leave full lo clutching position.

The tractor thus automatically responds in a smooth transition to asafe, lo clutch drive on both tracks whenever the operator releases bothsteering levers from a stopped position being maintained by the leversbeing held by him for full braking.

It takes conscientious effort, desirably so, for the operator to keepthe steering levers moving through and past the hydraulically detentedposition reached. The reason is that the second lost motion travel,return spring 322 is on the threshold of bottoming out in the positionshown in FIG. 5, and so all the force that is left in the way ofassistance which the operator has in overcoming the 290 psi pressure at270 against the right steer valve 44 is the force of the first returnspring 324 and intermediate spring 326.

SHIFT CYCLE COMPLETION

When the operator forces the steering levers and valves, e.g., the leftsteer valve 44-2 as shown in FIG. 7, into the neutral clutch-clutchposition N1 on the way to full hi clutching, the steering status calledfor is that the clutches and brake concerned are in disengagedcondition, the opposing hydraulic pressure in the intervalve chamber iszero, the intermediate, lost motion travel, return spring 326-2 isbottomed out, and the constantly effective return spring 324-2 has acountervailing force over the so-called weak spring pressing against themetering valve spool 226-2.

Therefore the left steer valve 44-2 in this example will, either with orwithout the operator's assistance, be moved by the force of thecountervalving spring 324-2 into the full hi clutching position as shownin FIG. 6.

Full hi clutching pressure of 290 psi (20 atmospheres) in the systemcompletes the shifting cycle, with the brake concerned being fullydisengaged and the hi clutch returned to fully engaged position.

GEARED STEERING SYSTEM

The foregoing system is referred to as a geared steer drive ortransmission system, or simply geared steering, in contrast to existingsingle drive systems having only a single steering clutch on each sideof the tractor. Design complications such as there might be are morethan offset because by contrast, one thing offered by the geared steersystem is an added speed for forward and an added speed for reverse ofthe tractor.

Another thing offered, significantly so, is that full hi clutching oneither side of the tractor attended by lo clutching on the opposite sideaffords a full power turn in the direction of that opposite side, i.e.,the tracks at the sides of the tractor are kept each one under fullpower throughout the execution of the turn. And with the added (high)speed available on the outside during short radius turns and pivotturns, the turns can be completed quicker.

SIMPLIFICATION

The design complications conceivable with an upgrading of single drivesteering-by-driving, to a geared steering system can be noted to beminimized with the latter system as herein taught.

One important reason for the simplification is that the hi clutch andbrake are siameze-connected by the steer valve concerned and that theabsence or presence of a single, unmodulated 290 psi control pressuresignal applied to the siameze hi clutch and brake via their common port174 inherently coordinates such clutch and brake for their similarlycushioned alternate engagement; to siameze pairs of intakes on manifoldsis accepted practice in the art, such as in engine inlet manifoldconnections, but to use a siameze connection on hydraulic cylindersinvolves a much different consideration. A similarly important reason isthat the lo clutch and brake are siameze-connected by the branches 204,206 and that a single control pressure signal incrementally modulated atand between 0 and 270 psi likewise inherently coordinates the siamese loclutch and brake, in common at 200.

An equally important reason how the simplification is herewithaccomplished resides in the direct internal hydraulic interconnectionsand valve consolidations realized from having the present coacting leftsteer and metering valve spools share a common bore, the coacting rightsteer and metering valve spools share a common bore, and the coactingtow and brake valve spools hereof share a common bore.

Variations within the spirit and scope of the invention described areequally comprehended by the foregoing description.

What is claimed is:
 1. Consolidated valving assembly means provided withhigh pressure, low pressure, and signal output conduits and controllablyconnecting the high and low pressure conduits to supply incrementallymodulated pressure signals in ranges therebetween and unmodulatedsignals to the signal output conduits, said assembly means having:acommon bore (190) hydraulically disposed for communicating with saidconduits; independently slidable, multi positionable hydropotentiometer(44) and satisfied-metering spools (226) in the common bore forestablishing therein signal and modulated pressures, respectively; saidhydropotentiometer spool having a 1st position constituting its homeposition and providing unmodulated pressure as an unmodulated 1stsignal, a 2d position constituting a moved position and providingunmodulated pressure as a 2d signal which is unmodulatable unless thespool is further moved, and a 3d position constituting a further moved,braking range threshold, position and providing modulated pressure as asignal which is further modulatable to a 3d signal with additional spoolmovement to introduce the braking range when the 3d signal isinaugurated, all for collectively affording a series of 1st, 2d and 3dsignals respectively supplied by the assembly means to the signal outputconduits thereof; first biasing means (324) associated with said borealong the line of travel of said hydropotentiometer spool, and beingpre-loaded and having an end engaged by the just-said spool in said 1stposition for unaugmented resistance to the spool to prevent it fromdiscontinuing the 1st signal; second biasing means (322) associated withsaid bore along the line of travel of said hydropotentiometer spool, andbeing pre-loaded and having an end engageable by the spool in a 2dposition in a clutching range, for substantially augmented resistance tothe spool to prevent it from modulating the 2d signal; and third biasingmeans (328) associated with said bore along the line of travel of saidhydropotentiometer spool, and being pre-loaded and having an endengageable by the spool in a 3d position at threshhold to a brakingrange, for fully augmented resistance to the spool to prevent it frominaugurating the 3d signal.
 2. In consolidated valving assembly meansprovided with high pressure, low pressure, and signal output conduits tosupply incrementally modulated pressure signals in ranges therebetweenand unmodulated signals to the signal output conduits, the improvementcomprising:a common, multi-valve bore (190) disposed for communicatingwith said conduits; a multipositionable selector valve spool (44) withreturn spring means in common bore controlled for movement therein bysaid return spring means (146) and by opposing pressure from anintervalve chamber (270) defined by and between the valve spool and anadjacent metering valve (226) in the common bore; said valve spoolbiased by the return spring means in an approaching direction of thecollapsing intervalve chamber through an intermediate second position(FIG. 5) toward a first main drive home position; passage means (268) insaid valve spool intercommunicating the intervalve chamber and a backpressure chamber (321) along the line of travel of the valve spool so asto afford a restricted escape path for the collapsing intervalve chamberfluid into lower pressure of the back pressure chamber as the spool insaid direction of biased movement approaches the second position; saidspool's second position corresponding to another position of drivedifferent from its first main drive, home position aforesaid; saidcombination further comprising means in said valve spool passage means(266) restricting the flow in said escape path, whereby the biased valvespool approaches the second position at an arrested rate of biasedmotion; and means (262, 264) responsive to the biased movement of thevalve spool to increase pressure in the back pressure chamber wherebythe return spring means will be balanced by the corresponding opposingintervalve chamber pressure by the time the valve spool reaches thesecond position, so as to be set hydraulically detented at rest thereat;and said return spring means including first biasing means (324)associated with said bore along the valve spool's line of travel,pre-loaded and having an end engaged by the spool in said first maindrive home position for unaugmented resistance to the spool to preventit from leaving the first position, and second biasing means (322)associated with said bore along the valve spool's line of travel, andbeing preloaded and having an end engageable by the spool in the seconddrive position, for substantially augmented resistance to the spool toprevent it from leaving the second position.